[oe-core][RFC 0/3] u-boot: Support SPL Verified Boot

[Klaus Heinrich Kiwi]

This patch series aims at extending U-Boot's verified boot support to
also include SPL signing.

The proposal is to some of the infrastructure (variables, functions)
used to sign the Kernel FitImage to more common locations, and then
essentially duplicate the method currently used to sign the Kernel
fitImage to also sign the U-Boot fitImage.

In the UBOOT_SIGN_ENABLE = "1" scenario, nothing really changes: The
Kernel fitImage is created, then signed, and the pubkey is added to
u-boot.dtb which is concatenated with the u-boot-nodtb.bin to create the
u-boot final image.

In case SPL_SIGN_ENABLE = "1", The U-Boot PN will take care of (re-)
creating the U-Boot fitImage (using custom .its script) after compile,
sign it, and contatenate the u-boot-spl.dtb (with the public key) with
u-boot-spl-nodtb.bin to create the final U-Boot SPl on deploy.

In case both UBOOT_SIGN_ENABLE and SPL_SIGN_ENABLE are set, the Kernel
PN will take care of creating and signing the U-Boot fitImage (becase we
need to also sign the FDT image containing the Kernel pubkey), and take
care of deploying it.

I tested all three scenarios using OpenBMC upstream, and although there
might be some areas of improvement (like deploying the new binaries and
symlinks with more useful names), it appears to work well.

One caveat is that when moving between the scenarios above, the user
might need to remove the tmp/ directory, since there could be a
collision for some of the files deployed into the images directory,
since the configuration may determine which PN does that.

Reviews, thoughts and comments are very very welcome,

Thanks,

 -Klaus